This invention relates to an electron gun comprising a section for generating at least one electron beam and a main lens system which, viewed in the propagation direction of the electron beam, has a first electrode, a final electrode and at least one intermediate electrode, each having at least one aperture allowing the electron beam to pass and being separated from each other by a gap with a chosen field strength, at least one of the gaps having the highest field strength, a main lens voltage being applied step-wise across said electrodes during operation so as to form an electron-optical focusing lens.
The invention also relates to a display device provided with an electron gun of the type referred to above.
An electron gun as described above is disclosed in European Patent Specification EP-B-0725972. The electron gun according to this specification comprises a main lens with at least one intermediate electrode between the first electrode and the final electrode, as viewed in the propagation direction of the electron beam. In more conventional electron guns, the main lens only has a first and a final electrode, which are usually referred to as the focus electrode and anode electrode, respectively. By adding intermediate electrodes, the main lens is distributed across a larger number of electrodes. For this reason, a main lens of this type is often referred to as a Distributed Main Lens (DML).
The separate electrodes of the main lens system in the known device are interconnected by means of a resistive voltage divider so that the main lens voltage is distributed step-wise across the electrodes during operation in order to reduce the magnitude of potential jumps in the main lens system. This leads to considerably improved lens properties as compared with more conventional guns in which the main lens voltage is entirely applied across only two electrodes. Notably, spherical aberrations can be adequately suppressed to relatively large electron beam currents without an increase of the mechanical lens diameter.
An electron gun of this type can be applied in, for instance, a conventional display device like a cathode ray tube (CRT). Such a display device comprises an evacuated envelope having a neck, a cone and a display window. The electron gun is situated in the neck part of the display device. The display screen is usually provided with electroluminescent material which is excited by the at least one electron beam from the electron gun. Examples are a monochromatic CRT, in which only one electron beam is present and also only one color of electroluminescent material, and the well-known color CRT which has an electron gun for generating three electron beams which, after having passed a color selection means, will excite three colors (e.g. red, green and blue) of electroluminescent material. Furthermore, a deflection unit for generating deflection fields for deflecting the at least one electron beam in the horizontal and in the vertical direction, thus scanning the entire display screen, is mounted around the cone part of the tube.
One of the performance items of an electron gun is its stray emission behaviour. In an electron gun, the different electrodes are separated by a gap. During operation, a voltage is applied to each electrode. As a result, a voltage difference is present across the gap between two adjacent electrodes, which leads to a chosen field strength between the adjacent electrodes. This field strength may give rise to the phenomenon which is referred to as stray emission. This may occur when the electrodes are not absolutely clean, as is the case with xe2x80x98loosexe2x80x99 particles that may get stuck to the electrodes, or when a small burr is left on the electrode. These particles or burrs on the electrodes may serve as a source for electron emission. At the edges of the apertures in the electrodes, the field strength is commonly higher, so that these edges may also serve as a source for electron emission. Electrons originating from such a source are emitted through a large spatial angle and are directed towards the electrode with the higher voltage. For this reason, they are referred to as stray electrons. Especially, stray electrons originating from the main lens may land on the entire screen where they unintentionally excite the electroluminescent material on the screen, causing a deterioration of the contrast performance of the display device.
It is an object of the invention to provide an electron gun which is constructed in such a way that it shows an improvement with respect to its stray emission behaviour, in comparison with existing electron guns with intermediate main lens electrodes.
According to the invention, an electron gun with which this object is realized is characterized in that, for each electron beam, the aperture of at least one of the electrodes following the first electrode is smaller than the aperture of said first electrode.
Most conventional electron guns are manufactured by using an inner reference system. This means that the several electrodes of the gun, separated by spacers, are mounted on pins. In this process, the anode electrode of the main lens is put on the pins as the first one, and the first electrode, being the one closest to the cathode, is mounted as the last one. During the beading process, two or more beading rods are used for assembling the beaded unit of the gun. In order to be able to remove this beaded unit from the mounting pins, it is necessary that the apertures of the consecutive electrodes show a non-increasing aperture size from the anode electrode to the first electrode. For this reason it is clear that, for more complex electron guns, the inner reference system has serious shortcomings. European Patent Specification EP-B-0376372 discloses a reference system which overcomes these shortcomings. By making use of the specially shaped outer contour of the electrodes, it is possible to use this for aligning the electrodes. For obvious reasons, this is referred to as the outside reference system. The use of such a reference system will be very beneficial for this invention, because it will be possible that one of the electrodes has smaller apertures than its preceding electrode; preceding here means closer to the cathode.
The invention is based on the recognition that, in an electron gun manufactured with the outside reference system, it is possible to make the apertures in at least one of the main lens electrodes smaller than those of at least one of the electrodes closer to the focus electrode (the first electrode). Decreasing the aperture size of an electrode causes more stray electrons to be intercepted by this electrode and, due to this, the stray emission behavior is improved.
A preferred embodiment of the electron gun according to the present invention is characterized in that, for each electron beam, the apertures of the electrodes following the gap with the highest field strength are smaller than the apertures preceding the gap with the highest field strength. Since the possible occurrence of stray emission is dependent on the field strength, the risk of stray emission is greatest between the electrodes with the highest field strength. Making the apertures of the main lens electrodes following this gap smaller will yield a better stray emission performance.
Another embodiment of the electron gun according to the present invention is characterized in that, for each electron beam, the apertures of consecutive electrodes decrease from the first electrode to the final electrode. In a focusing lens, the electron beam is normally already converging in the region where the electrodes are positioned. This implies that the electron beam diameter decreases from the first electrode to the final electrode. As a consequence, the apertures may decrease without sacrificing beam clearance, which is half the difference between aperture diameter and beam diameter.
A further embodiment of the electron gun according to the present invention is characterized in that, for each electron beam, the aperture of the final electrode is smaller than the apertures of the preceding electrodes of the main lens system. In such an embodiment, only the final electrode differs, which is an advantage for both electron-optical and mechanical reasons. The advantage with respect to the electron optical performance originates from the fact that the intermediate electrodes may be, for example, identical, leading to a partial cancellation of some main lens errors. Having identical intermediate electrodes is of course advantageous for mechanical reasons, because it will be possible to manufacture these from the same equipment.
The invention also relates to a display device provided with an electron gun according to the invention.